8 
BIRD-LIFE. 
together early, and rapidly become hollow, ready for the 
reception of air. The jaws or mandibles are both more 
or less moveable. They are without teeth, covered by 
a horny sheath, and are composed of a number of 
separate bones. 
The principal muscles are those which move the wing. 
In birds of prey the great pectoral muscles are very strong 
and massive. In the case of birds which do not fly, such 
as the Ostrich, these muscles, as well as the sternum, are 
very imperfectly developed, while those of the thighs, 
usually slight in most birds, are extremely powerful. 
Besides these, the muscles of the lower limbs merit notice 
on account of their gradual transmutation into the tendons 
which serve to move the feet and toes. It is also worthy 
of remark that certain of the tendons passing direct from 
the muscles of the thigh serve to bend both the knee and 
the toes, by which means the bird is able to sleep with 
bended knees, and thus, without conscious effort, firmly 
grasp a branch. The muscles of the tail are also very 
powerful, being used to elevate, depress, twist and 
turn that appendage. The muscles of mastication are 
more firmly constructed, the cutaneous muscles more 
effective than those of mammals. Very remarkable are 
the muscles which act on the skin of the wing, one of 
which extends between the body and the upper arm, and 
the other between the upper arm and the fore arm. The 
muscles of the abdomen are very insignificant, owing to 
the great extension of the sternum, while the diaphragm 
merits scarcely the name of a partition between the 
cavities of the breast and belly. All the muscles are 
capable of immense exertion. 
The digestive organs are very heterogeneous, varying 
much with the food of the different species. The mouth 
